A. Field of Invention
The present invention relates generally to oriented, polycrystalline, high-temperature superconducting ceramic oxides and, more particularly, to a process for preparing such superconductors from bilayer structures.
B. Description of Related Art
Superconductors are materials that, at sufficiently low temperatures and for sufficiently weak magnetic fields, have zero electrical resistivity. Absent electrical resistivity and the heat such resistivity creates, it would theoretically be possible for a superconductor to carry very large currents. As the current carried by a superconductor increases, however, a stronger and stronger magnetic field is self-generated until the critical magnetic field, above which the superconductor reverts to a normal conducting state with commensurate resistivity, is exceeded. Thus, there is a limit, the critical current density, to the amount of current a superconductor can carry.
High-temperature superconductors (those which superconduct above 77 Kelvin) are structurally related to the crystallographic family known as perovskites, ceramics that have a distinct atomic arrangement and that are found as common natural materials. Both natural and synthetic perovskites exhibit a wide range of electrical properties. One such property in the perovskites which are superconducting, the critical current density, is of great importance in the practical application of high-temperature superconductors.
Although the new, high-temperature superconductors have potential use at temperatures much higher than the extremely low temperatures required for superconduction by their older counterparts, they are inferior in other ways. Specifically, unlike older superconductors, the new, high-temperature superconductors lose their superconductivity under relatively small loads when polycrystalline and, consequently, are unable to carry sufficient current for practical applications. Researchers are now attempting to develop high-temperature superconductors having sufficiently high critical current densities for use in practical applications; no matter how high the critical temperature of a superconductor, it is practically useless unless able to support a useful flow of electric current.
A new, promising, high-temperature superconducting material is, for instance, the ceramic oxide YBa.sub.2 Cu.sub.3 O.sub.7-x (0&lt;x&lt;0.5). This material is designated the "123 superconductor" because the atomic ratios of Y:Ba:Cu are approximately 1:2:3. The 123 superconductor normally is a highly twinned orthorhombic phase, which is believed responsible for superconduction, and is closely related to a tetragonal, nonsuperconducting phase (YBa.sub.2 Cu.sub.3 O.sub.6). The only significant difference between the nonsuperconducting, oxygen-deficient phase and the superconducting phase is the presence in the superconducting phase of Cu--O--Cu chains along the b-axis direction on the a-c faces of the unit cell.
The 123 superconductor, like many of the new, high-temperature superconductors, is anisotropic meaning that crystals of the material conduct current better in one direction than in other directions. Some high-temperature superconductors can carry current thirty times more readily in one direction than in another direction. Because superconductivity is inhomogeneous on an atomic scale and localized to specific layers, current in the 123 superconductor flows poorly in one direction (the vertical direction, or c-axis of the unit cell, where c is the longest unit cell dimension) and easily in two directions (the a-b plane). In a polycrystalline material, the structure consists of an assembly of small, individual crystallites, whose relative orientation to each other may vary. Thus, orientation of the individual crystals in a bulk sample (as opposed to a single crystal) of the polycrystalline material is critical in determining the electrical properties.
Moreover, additional metallurgical characteristics of the various phases and their interfaces (such as phase boundaries, grain boundaries, defects, and impurities) may limit the current-carrying capacity. For example, grains tend to line up randomly in bulk materials and current has a difficult time flowing through randomly oriented grains. J. Mannhart, P. Chaudhari, D. Dimos., C. C. Tsuei & T. R. Mc Guire, Critical Currents in [001]Grains and Across Their Tilt Boundaries in YBa.sub.2 Cu.sub.3 O.sub.7 Films, Phys Rev. Lett. 61 2476 (1988). Impurities (such as the carbonates which form easily in Ba-containing compounds) lying between grains in bulk samples also make it harder for current to pass from grain to grain, which lowers the critical current density. Again, the growth orientation of the crystals in a polycrystalline material is important.
With proper, preferred orientation or texture of the crystals, a superconductor such as the 123 superconductor develops a path or track through its internal structure along which electrons are free to pass. The preferred orientation or texture has been shown for very thin films, those less than 1 micrometer thick, to have the c-axis perpendicular to the film surface. Such an orientation produces high critical current. Thus, the metallurgical formation of the material directly affects critical current density. Normally, when a thick 123 superconductor coating is deposited on a substrate material, as by plasma spraying, and is given a conventional heat treatment, the crystals form a random polycrystalline array.
For reasons discussed above, high critical current density can be obtained in the 123 superconductor only when there is a continuous current path along the base plane of the unit cell. Thus, a necessary condition to attain a high critical current density in polycrystalline superconducting ceramic oxides such as the 123 superconductor is to achieve the proper orientation in such materials. Specifically, the 123 superconductor must have an orientation in which the c-axis of the unit cell is oriented normal to the substrate material (and to the plane of the superconductor film). This condition is necessary but not sufficient to achieve high critical current density. It is also required that the remaining boundaries between grains have no greater misfit than is described by semi-coherent theory (i.e., there must be a significant fraction of coherent boundaries) and that the boundaries be free of impurities, inclusions, and all other foreign matter that hinders the super current transport.
A number of processes have been developed to produce oriented superconducting ceramic oxides like the 123 superconductor. One method consists of introducing an additive, Ag.sub.2 O, in powder form to the 123 superconductor ceramic oxide powder during processing. A modest increase in the critical current density by a factor of up to five is obtained. The increase is attributed to an increase in the orientation or texture (grain alignment) of the polycrystalline samples. M. K. Malik, V. D. Nair, A. R. Biswas & R. V. Raghavan, Texture Formation and Enhanced Critical Current in YBa.sub.2 Cu.sub.3 O.sub.7, Appl. Phys. Lett. 52 (18), 1525-27 (May 2, 1988).
In another method, oriented thin films of 123 superconductors, with crystallites having the c-axis of the unit cell normal to the plane of the superconductor film, have been produced by sputtering and evaporation onto substrates such as SrTiO.sub.3 and MgO. P. Chaudhari, R. H. Koch, R. B. Laibowitz, T. R. Mc Guire & R. J. Gambino, Critical Current Measurements in Epitaxial Films of YBa.sub.2 Cu.sub.3 O.sub.7-X Compound, Phys. Rev. Lett. 58, 2684 (1987); D. S. Yee, R. J. Gambino, M. Chisholm, J. J. Cuomo, P. Madakson & J. Karasinski, Critical Current and Texture Relationships in YBa.sub.2 Cu.sub.3 O.sub.7 Thin Films, AIP Conf. Proc. No. 165, at 132 (J. M. E. Harper, R. J. Colton & L. C. Feldman eds. 1988) (from Topical Conference on Thin Film Processing and Characterization of High T.sub.c Superconductors, Anaheim, Calif., Nov. 8, 1987). The polycrystalline substrates which are typically used in plasma spray deposition processes, however, do not afford the possibility of such epitaxial growth.
In fact, the substrate or base layer must be carefully matched with the superconductor in existing thin film applications. Ideally, the substrate's crystalline structure should be identical to that of the superconductor so that the thin film can be deposited with a minimum of flaws and dislocations. Moreover, when the substrate interdiffuses with the superconductor during the annealing phase of processing, the superconductivity may degrade. Degradation may result because superconductors are very sensitive to an influx of foreign atoms. For example, interdiffusion can completely destroy superconductivity if the substrate contains even just a few percent silicon.
Melt-textured growth, or directional spherulitic growth, processing represents still another method available to achieve orientation. Conventional processing of ceramic oxide superconductors includes a sintering step in which the superconductor is fired at relatively low temperatures (about 700.degree. C. for the 123 superconductor) to sinter individual grains together. The sintered material exhibits a somewhat porous, fine-grained, and randomly oriented microstructure.
Using melt-textured growth, in contrast, the superconductor is melted at very high temperatures (about 1050.degree.-1200.degree. C. for the 123 superconductor) and slowly cooled to obtain directional solidification. The resulting material consists of long, needle-like crystals and grains that are preferentially aligned in roughly the same direction. The material is also more dense and shows enhanced decomposition of unwanted impurities relative to the sintered material. These considerable microstructural changes, caused by the melt-textured growth process, are reflected in a significantly increased critical current density. S. Jin, T. H. Tiefel, R. C. Sherwood, M. E. Davis, R. B. van Dover, G. W. Kammlott, R. A. Fastnacht & H. D. Keith, High Critical Currents in Y-Ba-Cu-O Superconductors, Appl. Phys. Lett. 52(24), 2074-76 (June 13, 1988).
The melt-textured growth process produces large grain growth and induces significant (near 100%) densification. Such properties tend to cause microcracking during the processing step of cooling, because the material cannot absorb the stress of the change in crystalline structure as YBa.sub.2 Cu.sub.3 O.sub.6 converts into YBa.sub.2 Cu.sub.3 O.sub.7. Problems also arise in the final processing step: oxygenation is made more difficult by the dense microstructure. Both of these undesirable features limit the critical current density possible and adversely affect other properties as well.
One known heat treatment, described herein as the "pinch-off" procedure (zone melting of a film through a restriction), is of importance because it allows polycrystalline or amorphous films to recrystallize as single crystal (monocrystalline) films. H. A. Atwater, H. I. Smith & M. W. Geis, Orientation Selection by Zone-Melting Silicon Films Through Planar Constrictions, Appl. Phys. Lett. 41 (8), 747-49 (Oct. 15, 1982); H. I. Smith, M. W. Geis, C. V. Thompson & H. A. Atwater, Silicon-On-Insulator by Graphoepitaxy and Zone-Melting Recrystallization of Patterned Films, J. Crystal Growth 63, 527-46 (1983). This procedure is best explained by reference to FIGS. 1 and 2. Initially, the polycrystalline material, shown deposited on substrate 10 has relatively fine grains 12. By etching the material, using standard techniques (e.g., chemical, ion milling, and the like), gates 14 are formed. A narrow annealing (or "hot") zone 16 then is moved along the film surface (from left to right in the figures) and the annealing temperature is adjusted so the grains of the polyscrystalline film grow (while in the annealing zone) into larger grains 18; the etched gates are formed so that the width of the gate opening or pinch-off zone 20 is slightly smaller than the grain size obtained during annealing.
The region with the larger growing grains is moved along the film surface along with the annealing zone. The gates restrict the number of grains, however, that grow and propagate along with the annealing zone. Thus, only a few (but larger) grains will propagate as the annealing zone passes the first gate. Fewer and fewer (larger and larger) grains propagate as the annealing zone passes subsequent gates until, finally, only a single (large) grain grows. The result is a single crystal film, 22. Thus, the heat treatment applied is seen to have a marked effect on the metallurgical formation of the material and, hence, on the critical current density.
To overcome the shortcomings of existing processes, a new process for preparing oriented, polycrystalline, superconducting ceramic oxides--and specifically the 123 superconductor--is provided. An object of this invention is to provide a process by which superconductor coatings having significantly improved critical current densities can be made. A related object is to improve the c-axis orientation of the material.
Another object is to provide the improved orientation without incorporating foreign additives such as Ag.sub.2 O during the process. It is still another object of the present invention to provide a texturing process which is not a sensitive function of the cooling rate and which does not interfere with required processing steps such as oxygenation. It is a further object to transform amorphous or random polycrystalline films on polycrystalline substrates into oriented films (either polycrystalline or monocrystalline) while, at the same time, preventing microcracking.
Yet another object of this invention is to provide a process of making oriented superconducting materials having high critical current densities which is independent of the substrate material and its crystal orientation or texture.
Still another object is to provide an oriented, superconducting coating which can itself act as a base layer to facilitate the deposition of another oriented, high-temperature, superconducting oxide by conventional methods and which will prevent contamination of the new superconducting overlayer by providing a barrier to interaction with the underlying substrate as well as by matching the composition of the oriented base layer to that of the overlayer.